Abstract

In order to improve the cycling performance of LiMn2O4 based cathode materials, we have synthesized a new composition LiCo0.4Al0.1Mn1.5O4 by sol-gel method. In the path of the material synthesis, citric acid was added to serve as a binding agent and a gelling agent respectively, followed by calcinations at 850°C for 12 h. The synthesized material was well characterized by TG/DTA, XRD, FTIR, EPR, SEM, electrical and electrochemical tests. It was found that LiCo0.4Al0.1Mn1.5O4 powder has an ordered cubic spinel phase (space group Fd3m) and exhibits good rate capability. The electrical and electrochemical characterization was carried out in CR-2032 coin type cell configuration. The material delivers an initial discharge capacity of 48 mAhg-1 between 3.5 and 4.9 V at a C/10 rate and subjected for more than 30 cycles. The electrochemical behavior is well supported with impedance data.

Keywords

Introduction

Considering the development of renewable energy sources and the large scale power requirement during peak hours from the electrical grids, energy storage devices are very important. In addition to ever growing portable electronic devices, the use of Li-ion battery (LIB) is expanding even for large scale applications such as hybrid or plug-in electric vehicles. Therefore, it is important to develop high performance LIBs having efficient capacity retention over long cycles. For this purposes, both anode and cathode materials are being modified to improve their respective performances. For instance, different cathode materials based on LiFePO4 and LiMn2O4 structures with various dopants have been studied [1-10] as alternatives to LiCoO2. Among these, LiMn2O4 based materials find special attention due to the abundance, cost and environmental compatibility of manganese [11,12]. Even though LiMn2O4based materials exhibit scrupulous capacity, they show severe capacity fading upon cycling due to the formation of Mn2+ ions through John-Teller active Mn3+ ions especially at elevated temperatures, 50-60°C [13-15]. In order to combat this stability issue, partial substitution of Mn with various metal ion (M=Co, Gd, Zn, In, Fe, Au, Ni, Cr, Mg, Sn, Al and B) have recommended [1- 6]. This is based on the idea that the dopant ions increase the average valence state of Mn to be higher than +3.5. This will also stabilize the LiMn2O4 framework structure by strong metal-oxygen bonding of the substituted metal ion [16,17].

LiMn2O4 was first synthesized by heating a mixture of lithium carbonate and manganese oxide at 850°C in air [18]. The lithium ions could be easily de-intercalated from the spinel LiMn2O4 structure to form λ-MnO2 by a chemical process [19]. Hence, it was used as a cathode material [20]. Various LiMn2O4 based samples such as LiMgxSnyAlzMn2−x−y−zO4 [1], LiM0.02Mn1.98O4 (M=Zn, In, Co, Ni) [2], LiMxMn2-xO4 (M=Fe, Au) [6], LiMn2-x-yNixCryO4 [3], LiZnxNi0.5−xMn1.5O4 [4], and Li1.15Mn1.96Co0.03Gd0.01O4+δ [5] have shown better capacity retentionS compared to pristine LiMn2O4. For instance, LiM0.02Mn1.98O4 showed initial discharge capacity of 113, 134, 124, and 127 mAhg-1 and the total capacity loss as 22%, 9%, 11% and 12%, respectively, for M=Zn, In, Co and Ni after 50 cycles [2]. LiMn2−x−yNixCryO4 showed initial discharge capacity of 128 mAhg-1 and the capacity loss as 10% after 230 cycles [3].

Similarly, the initial discharge capacity of Li1.15Mn1.96Co0.03Gd0.01O4+δ is 128.1 mAhg-1 and the total capacity loss is 1% after 100 cycles [5]. In our previous study on LiNi0.4M0.1Mn1.5O4, an improved capacity was found for M=Al when compared to M=Bi [21]. Therefore, it is of interest to study the Al- and Co-doped samples such as LiCo0.4Al0.1Mn1.5O4 wherein the variable oxidation state of Co can impact on the average valence state of Mn.

Many soft chemical methods [1,21-23] have been developed to synthesize LiCoO4 and LiMn2O4 powder instead of solid-state reactions. Because the materials synthesized by solid-state method often lead to inhomogeneties, irregular morphology and broad distribution of particle sizes. Thus, the sol-gel preparation and characterization of LiCo0.4Al0.1Mn1.5O4 sample as cathode material for LIB is reported here.

Materials and Methods

LiCo0.4Al0.1Mn1.5O4 in the powder form was prepared by taking the stoichiometric amounts of lithium hydroxide, aluminium hydroxide, cobalt acetate and manganese acetate in deionized water with a drop of nitric acid. Citric acid (ligand) is added in such a way to keep a metal ion to ligand ratio of 1:1. The mixture was stirred vigorously at 90°C till it formed a viscous liquid, gel. It was dried at 150°C for few hours and after grinding to fine powder in agate mortar, it was heated to 350°C for 4 h to obtain an intermediate compound. This was calcined at 850°C for 12 h to form LiCo0.4Al0.1Mn1.5O4.

X-ray diffraction (XRD) pattern was obtained using Cu Kα radiation (λ=1.542 Å) with a Ni-filter. Rietveld refinement of the XRD pattern was done using POWPREF programme. Morphology (SEM, HITACHI S-3000H) and the elemental composition of the sample was examined by recording energy dispersive X-ray (EDXA) spectra. FT-IR spectrum was recorded on a Nicolet 5DX-FTIR spectroscopy with KBr pellet. Electron paramagnetic resonance (EPR) was studied by a Bruker EMX plus X-band spectrometer.

The electrical and electrochemical studies were performed with a CR-2032 coin type cells assembled in an Ar-filled glow box (mBRAUN MB200G) having moisture and oxygen levels at less than 0.1 ppm. The LiCo0.4Al0.1Mn1.5O4 powder (80 wt%), SP–carbon (Timcal, 15 wt%) and PVdF (5 wt%) were ground with a drop of NMP solvent to form a homogeneous slurry. This slurry coated on Al–foil was dried and cut into circular discs to use in the cell assembly as cathode. Li–foil (thickness: 0.70 mm) was used as anode. The electrodes were separated by a Celgard® 2400 (polypropylene) soaked in the 1M solution of LiPF4 in ethylene carbonate-dimethyle carbonate (EC/DMC, 1:1) electrolyte.

A cyclic voltammogram was recorded using Autolab PGSTAT 302n for a CR-2032 coin type cells which formed a three electrode cell - the working electrode i.e., the active cathode material was combined with a Li-foil used as reference and counter electrode. A scan rate of 0.1 mVs-1 between 3.5-4.9 V vs. Li was set.

Lithium intercalation behavior was evaluated between 3.5 and 4.9 V at C/10 rate by an Arbin multichannel charge-discharge instrument (BT2000).

Results and Discussion

Formation and structural studies

The material formation kinetics has been done by using TG/ DTA (Figure 1). In TG curve, we can observe three regions of weight loss: The first region (RT-180°C) is due to loss of adsorbed water molecules, second region (180 to 330°C) is due to the loss of crystalline water molecules and third region (330-420°C) is ascribed to the decay of acetates and hydroxides leading to the formation of end product with H2O and CO2 as byproducts. This weight loss in three regions is in well agreement with the expected weight loss. Based on TG data, the material formation temperature is about 420°C. Also the DTA curve shows three exothermic peaks at about 230,350 and 400°C corresponding to decay of crystalline water and the acetates/hydroxides, respectively.

Rieitveld refined powder XRD pattern of the LiCo0.4Al0.1Mn1.5O4 is shown in Figure 2 which confirms the well crystallized cubic (normal spinel) structure with space group of Fd3m (JCPDS file no. 89-8325). The lattice parameter (a=b=c=8.1712(1) Å and α=β=γ=90°) are in agreement with literature values [6,24]. The average crystallite size d was calculated to be about 76 nm using Scherer formula. The XRD patterns showed a single phase nature of material with no impurity. In the structure, the corresponding atoms are located in their own sites and the observed peaks are well coordinated with the calculated one with high reliability factor (%): Rp=2.90, Rwp=3.85, RF2=3.94 χ2=2.677.

FT-IR spectra of the LiCo0.4Al0.1Mn1.5O4 shown in Figure 3. Richardson et al. [25] has shown the symmetry of parent cubic spinel LiMn2O4. Based on group theory analysis, there are four infrared active vibrations such as (i) 2A2u+2Eu for the D5 3d group [26], (ii) Trigonally distorted alternating layers of LiO6 and MO6 octahedra are present in the crystal structure of LiMO2 layered oxides. (iii) The Wychoff sites 3(a) and 3(b) consist of transition metal cations (i.e., Co, Ni, Mn) and (iv) lithium ions [27]. The band at around 520 cm-1 and 630 cm-1 has been assigned to Li-O and Li-Mn-O stretching vibration respectively. Li-O stretching vibration and Li-Mn-O stretching vibration indicates the formation trigonally distorted alternating layers of LiO6 and MnO6 octahedra [27], which in good agreement with previous report of Thirunakaran et al. [28] and Nayaka et al. [21]. The broadening of FTIR bands may be due to the cation mixing in the crystal layers.

Figure 3: FT-IR spectra of LiCo0.4Al0.1Mn1.5O4 calcined at 850°C.

SEM image of LiCo0.4Al0.1Mn1.5O4 (Figure 4) shows the polydispersed nature of particles and their agglomeration. All the constituent elements are present in expected levels as shown in EDXA analysis (except Li which cannot be detected by EDXA).

Figure 4: (a) SEM image and (b) EDXA spectra of LiCo0.4Al0.1Mn1.5O4.

Electron paramagnetic resonance studies

The cation distribution and changes in the Mn4+ environment during lithium extraction/ insertion can be monitored by EPR spectroscopy. The effective g–factor and the intensity of the sharp signal depend on the synthesis conditions [29]. The sample LiCo0.4Al0.1Mn1.5O4 contains two magnetic ions: Co2+ (S=3/2) and Mn4+ (S=3/2). Because of its odd number of d-electrons, Co2+ is a Kramers ion and its EPR spectra is very sensitive to the interactions with the environment [30]. The paramagnetic resonance detected here could be attributed to the presence of octahedral Mn4+ ions that carry a half–integer spin (S=3/2) and are then EPR–active. EPR spectra recorded for as prepared sample shows (Figure 5) the sharp signal centered at gyromagnetic factor, g ≈2 with Lorentzian curve with a peak to peak width (ΔHpp) of 348 mT [31]. This is in accordance with the previous studies of Stoyanova et al. [32] and Nayaka et al. [21]. It is an indicative of the ordered nature of the sample having paramagnetic interaction among 16d site ions in the spinel structure [33,34].

Figure 5: EPR spectra of the LiCo0.4Al0.1Mn1.5O4 calcined at 850°C.

Electrical and electrochemical characterization

Ac-impedance is a powerful technique to study the kinetics of lithium de/insertion process [35]. Based on the Nyquist plot of LiCo0.4Al0.1Mn1.5O4 (Figure 6), recorded in the pristine state, the charge-transfer resistance was found to be about 400 Ω. These values indicate the electrode-electrolyte interface layer after charge-discharge cycle [36]. The pristine sample show high frequency semi circle, which represents the migration of Li+ ions at the electrode-electrolyte interface [37,38]. A straight line in low frequency region corresponds to the charge–transfer process. The resistance component, Rs, arise from the electrolyte and other cell constituents. The broad phase–transition peak here shows that the sample particles are not highly oriented [36,39]. An equivalent circuit describing the Randles model (inset of Figure 6) is used for fitting experimental data. Rs is the ohmic resistance, Qc is the constant–phase element, Rt is the charge transfer resistance of the electrodes and Zw is the Warburg impedance. Constant phase element has been used to provide accommodation for capacitor imperfections. The calculated values fitted well with the experimental values.

Figure 7 shows the cyclic voltammogram of LiCo0.4Al0.1Mn1.5O4 at C/10 rates. It is evident from the figure that the main anodic and cathodic peaks are observed around at 3.86/3.64 V and 4.17/3.79V vs. Li suggestive of lithium intercalation and deintercalation processes in to the spinel LixMn2O4(x<1) lattice through a two step process [40]. The redox performance observed here is in agreement with earlier reports [21,41].

Figure 7: Cyclic voltammogram of Li/LiCo0.4Al0.1Mn1.5O4 at a scan rate of 0.1 mVs-1.

Figure 8 shows the charge–discharge profile of LiCo0.4Al0.1Mn1.5O4/ LiPF6 (EC+DMC)/Li coin cell for first cycle at a constant current rate of C/10 with a minimum current limit of 0.11 mA between 3.5 and 4.9 V. The capacity (C, in mAh) of the cathode material is calculated from its theoretical capacity (i.e., Qtheo=149.208 mAhg-1) multiplied by its weight (7.92 mg). Figure 9 shows the galvanostatic cycling behavior of pristine LiCo0.4Al0.1Mn1.5O4. Coin cell delivered an initial discharge capacity of 48 mAhg-1 and at the end of 30 cycles it is 45 mAhg-1. Myung et al. [42] suggested that doping with Al suppresses the capacity fading of the LiMn2O4 based cathodes. Recently it has been demonstrated that Al is one of the dopant, improves the cycling performance of the LiMn2O4 based cathodes [8,21]. This is attributed to decrease in Mn3+ composition in the LiMn2O4 framework. The result here indicates the suppression of Jhan-Teller distortion. In the present case, we are not able to get good specific capacity compared to previous studies which may be due to the dissolution of active material in the electrolyte wherein HF is easily formed as LiPF6 was used as electrolyte salt [26], the decrease in specific rate capability may be attributed to an increase of electrode polarization during cycling [43], and it arises due to its incompatibility of cathode material with current collector and also the electrolyte. The electrochemical performance of electrode materials depends not only on structural characteristics but also on their formation history.

Conclusion

LiCo0.4Al0.1Mn1.5O4 material in the powder form was successfully prepared by citric acid assisted sol-gel method and characterized by XRD, FTIR, SEM and TG/DTA. The high frequency semicircle observed in the Nyquist plot is ascribed to the Li-ion migration throughthe interface from the surface layer of the particles to the electrolyte. The galvanostatic charge/discharge curves for showed reasonably good capacity retention (48 mAhg-1) at the end of 30 cycles Nevertheless, the detailed studies are needed to modify the sample characteristics to obtain the significant capacity retention.

Acknowledgement

One of the authors (GP Nayaka) gratefully acknowledges the Kuvempu University for financial support and Dr. Kenchappa R and Dr. Viswanath R for their help during this research work.